Recently it has been discovered that colloidal boehmite (alpha alumina monohydrate) when formed into a gel or gel-like paste, and seeded with ultrafine (less than 0.1 micron) alpha alumina particles, can be converted, by relatively low temperature firing into alpha alumina bodies of extremely fine crystal size (0.5 microns and finer) and substantially full density. The final density can be less, if desired, when shorter firing time or lower temperature or both are employed.
The final product alumina may be in the form of shaped bodies, such as tubes for filtration and the like, high desnity substrates for electronic purposes, wear resistant or refractory parts, and abrasive grits. The alumina may also form a matrix for bonding other refractory abrasives, or wear resistant materials.
One problem with the commercial development and use of such products is the relatively high cost of the suitable colloidal boehmites now available.
Current production of colloidal boehmite in industry stems from two main routes:
1. Aluminum alkoxide (or other Al-organic) hydrolysis PA1 2. Neutralization of sodium aluminate liquors
The first method relies on aluminum powder (of high purity). In the past the boehmite generated was actually a by-product of the Ziegler process for production of linear alcohols and hence production was limited by the alcohol market. However, it is possible to form the alkoxide and then recycle the alcohol to eliminate this dependence on the alcohol market. Clearly, the economics of this process depend largely on the price of aluminum powder, a relatively expensive commodity.
The second technique requires precipitating the boehmite (or precursor) from a solution of sodium aluminate. This results in some loss of reagents and requires that extensive washing of the precipitate be done to eliminate sodium salts which are generally deleterious to the end product application. While precipitation from sodium aluminate itself is not a costly procedure, the loss of reagents and extensive washing (filtering) makes this process relatively expensive as well.